1. Field of the Invention
The present invention relates generally to methods of acoustic measurement and determination of subsurface parameters in a wellbore and its surrounding formation.
2. Background Art
The oil and gas industry uses various tools to probe the formation penetrated by a borehole in order to locate hydrocarbon reservoirs and to determine the types and quantities of hydrocarbons. Among these tools, sonic tools have been found to provide valuable information regarding formation properties. In sonic or acoustic logging, a tool is typically lowered into a borehole, either after the well has been drilled or while the well is being drilled, and sonic energy is transmitted from a source into the borehole and surrounding formation. The sonic waves that travel through the borehole and formations are then detected with one or more receivers. Conventional sonic tools for this purpose are described in U.S. Pat. Nos. 5,852,587, 4,543,648, 5,510,582, 4,594,691, 5,594,706, 6,082,484, 6,631,327, 6,474,439, 6,494,288, 5,796,677, 5,309,404, 5,521,882, 5,753,812, RE34,975 and 6,466,513.
Acoustic waves are periodic vibrational disturbances resulting from acoustic energy that propagates through a medium, such as a subsurface formation. Acoustic waves are typically characterized in terms of their frequency f=V/λ (V is the speed of sound, λ is the wavelength), amplitude, and speed of propagation (Velocity, V).
An acoustic source in a fluid-filled borehole generates headwaves as well as relatively stronger borehole-guided modes. A standard sonic measurement system consists of placing source and receivers inside a fluid-filled borehole. The source is configured in the form of either a monopole, or a dipole, or a quadrupole source. The source bandwidth typically ranges from a 0.5 to 20 kHz. A monopole source generates primarily the lowest-order axisymmetric mode, also referred to as the Stoneley mode, together with compressional and shear headwaves. In contrast, a dipole source primarily excites the lowest-order flexural borehole mode together with compressional and shear headwaves. A quadrupole source primarily excites the lowest order quadrupole mode together with compressional and shear headwaves.
With reference to FIG. 1, a pulse 1 from a monopole source 32 travels through the fluid in the wellbore 33 at a speed Vf. This wave is a (fluid) compressional wave or P-wave. Some of the P-waves encounter the wellbore wall at a critical angle, ≦θi, that allows the wave to transmit into the formation and to refract as pulse 34 along the longitudinal wellbore axis. The critical incidence angle of the fluid compressional wave is θi=sin−1(Vf/Vp). There are two wave types that propagate along the axis through the surrounding formation, P-waves (with velocity Vp) and shear waves or S-waves (with velocity Vs). As the compressional and shear waves travel along the interface, it radiates waves 35 (S-waves being converted back to P-waves) back into the fluid that can be detected by the receiver array 36.
In fast formations (Vs>Vf), the shear headwave can be similarly excited by a fluid compressional wave at the critical incidence angle θi=sin−1(Vf/Vs). In a homogeneous and isotropic model of fast formations, compressional and shear headwaves can be generated by a monopole source placed in a fluid-filled borehole for determining the formation compressional (Vp) and shear wave (Vs) speeds. While FIG. 1 shows only two receivers it is understood by those skilled in the art that there may be more than two receivers. Any initial P-wave from the transmitter that has an incidence angle relative to the wellbore wall greater than the angle required for penetration is reflected, and the wellbore wall acts as a wave guide. Waves that travel along the interface of the drilling fluid and the borehole wall are referred to as Stoneley waves with a velocity Vst.
It is well known that refracted shear headwaves are not detectable in slow formations (where the shear wave velocity Vs is less than the borehole-fluid compressional velocity Vf) with receivers placed in the borehole fluid. In slow formations, formation shear velocities are generally obtained either from the low-frequency asymptote of flexural dispersion (using dipole transmitters) or through inversion of the quadrupole mode signals. There are standard processing techniques for the estimation of formation shear velocities in either fast or slow formations from an array of recorded dipole or quadrupole waveforms.
Acoustic properties of interest for formations may include compressional wave speed in the formation (Vp), shear wave speed (Vs), Stoneley or borehole modes, and formation compressional slowness (1/Vp). Additionally, acoustic images may be used to depict borehole wall conditions and other geological features away from the borehole. These acoustic measurements have applications in seismic correlation, petrophysics, rock mechanics and other areas.
Recordings of acoustic properties as functions of depth are known as acoustic logs. Information obtained from acoustic logs may be useful in a variety of applications, including well to well correlation, porosity determination, determination of mechanical or elastic rock parameters to give an indication of lithology, detection of over-pressured formation zones, and the conversion of seismic time traces to depth traces based on the measured speed of sound in the formation. As an example, the permeability of the formation around the wellbore can be extracted by analysis of the Stoneley wave characteristics, in particular the amplitude of the Stoneley waves.
A typical sonic log (a type of acoustic log) can be recorded on a linear scale of compressional slowness (1/Vp) versus depth in the borehole, and is typically accompanied by an integrated-travel-time log in which each division indicates an increase of one microsecond of the total travel time period.
Various analysis methods are available for deriving formation properties from the sonic log data. Among these, the slowness-time-coherence (STC) method is commonly used to process the monopole sonic signals for coherent arrivals, including the formation compressional, shear, and borehole Stoneley waves. See U.S. Pat. No. 4,594,691, which is incorporated by reference in its entirety, and Kimball et al., Geophysics, Vol. 49 (1984), pp. 264-281.
An example of a logging device that has been used to obtain and analyze sonic logging measurements of formations surrounding a borehole is called the Dipole Shear Sonic Imager (DSI™), and is of the general type described in Harrison et al., “Acquisition and Analysis of Sonic Waveforms From a Borehole Monopole And Dipole Source For The Determination Of Compressional And Shear Speeds And Their Relation To Rock Mechanical Properties And Surface Seismic Data,” Society of Petroleum Engineers, SPE 20557, 1990. In conventional use of the DSI™ logging tool, one can present compressional slowness Δtc (1/Vp), shear slowness, Δts (1/Vs), and Stoneley slowness, Δtst (1/Vst), each as a function of depth, z. The Stoneley slowness is estimated from the STC algorithm using a bandpass filtered (0.5 to 1.5 kHz) Stoneley waveforms.
Shear moduli are constants derived from the ratio of stress to strain in a formation. These constants relate the force exerted on a formation (stress) to the degree of permanent deformity (strain) caused by this force, and can be used as a measure of elasticity of the formation. U.S. Pat. No. 6,611,761 describes a technique for obtaining radial profiles of fast and slow shear slownesses using the measured dipole dispersions in the two orthogonal directions that are characterized by the shear moduli c44 and c55 for a borehole parallel to the X3-axis in an orthorhombic formation. U.S. Pat. No. 6,714,480 describes a technique for estimating the horizontal shear modulus c66 of an orthorhombic or TI-formation using the zero frequency intercept of the Stoneley dispersion that yields the tube wave velocity (Vst).
During a drilling operation, the drilling action and pumping of the fluids may damage the formation or introduce stress in the formation in the near wellbore region. Mechanical damage or stress near the wellbore may present trouble to a driller, such as causing tools to stick or slip. Thus, it is desirable to have a qualitative detection of near-wellbore alterations using measurements while drilling to provide real-time input to the wellbore stability model and to help identify problem zones ahead of time so that the driller can deal with issues before they become costly. Although sonic or acoustic measurements can provide information about formation mechanics and stress, such conventional measurements are susceptible to variations in borehole-formation parameters. Therefore, a need remains for methods that can provide qualitative detection of near-wellbore alterations with less sensitivity to variations in borehole-formation parameters.